Comparison of ZnO nanostructures grown using pulsed laser deposition, metal organic chemical vapor deposition, and physical vapor transport

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1 Comparison of ZnO nanostructures grown using pulsed laser deposition, metal organic chemical vapor deposition, and physical vapor transport V. E. Sandana a Nanovation, 103B Rue de Versailles, Orsay, France; Center for Quantum Devices, Northwestern University, Evanston, Illinois 60208; and Department of Irradiated Solids, Ecole Polytechnique, Palaiseau, France D. J. Rogers and F. Hosseini Teherani Nanovation, 103B Rue de Versailles, Orsay, France R. McClintock, C. Bayram, and M. Razeghi Center for Quantum Devices, Northwestern University, Evanston, Illinois H.-J. Drouhin and M. C. Clochard Department of Irradiated Solids, Ecole Polytechnique, Palaiseau, France V. Sallet GEMAC, 45 avenue des Etats-Unis, Versailles, France G. Garry Thales Research & Technology, Route Départementale 128, F Palaiseau, France F. Falyouni GEMAC, 45 avenue des Etats-Unis, Versailles, France Received 8 December 2008; accepted 27 April 2009; published 28 May 2009 This article compares the forms and properties of ZnO nanostructures grown on Si 111 and c-plane sapphire c-al 2 O 3 substrates using three different growth processes: metal organic chemical vapor deposition MOCVD, pulsed laser deposition PLD, and physical vapor transport PVT. A very wide range of ZnO nanostructures was observed, including nanorods, nanoneedles, nanocombs, and some novel structures resembelling bevelled nanowires. PVT gave the widest family of nanostructures. PLD gave dense regular arrays of nanorods with a preferred orientation perpendicular to the substrate plane on both Si and c-al 2 O 3 substrates, without the use of a catalyst. X-ray diffraction XRD studies confirmed that nanostructures grown by PLD were better crystallized and more highly oriented than those grown by PVT and MOCVD. s grown on Si showed relatively poor XRD response but lower wavelength emission and narrower linewidths in PL studies American Vacuum Society. DOI: / a Electronic mail: sandana@nanovation.com I. INTRODUCTION ZnO is a remarkable multifunctional material with a distinctive set of properties, including a direct bandgap of 3.37 ev, high transparency over the visible spectrum, a very wide range of possible conductivities, and a strong piezoelectric response. Thus ZnO has many established and emerging applications including varistors, light emitting diodes 1 LEDs and surface acoustic wave devices. 2 Nanostructuration of ZnO 3 further extends the range of potential applications by augmenting the basic property set with phenomena unique to the quantum world. 4 Indeed, nanostructured ZnO has become a huge research topic with more publications in 2008 than even carbon nanotubes. 5 There are many reasons driving this interest, including the unique property set of ZnO, 6 the ease of fabrication of ZnO nanostructures with a wide range of techniques, 7 the wide range of emerging and potential applications, 8 the biocompatibility of ZnO, 9 and the enormous family of nanostructures exhibited by ZnO probably the largest of any nanomaterial 6,10. Although ZnO has been grown with a vast range of different techniques, direct comparison of the properties of nanostructures grown with different methods is lacking in the literature. This article compares the forms, crystallographic properties, and optical properties of ZnO nanostructures prepared using three common growth processes: metal organic chemical vapor deposition MOCVD, pulsed laser deposition PLD, and physical vapor transport PVT. Three different techniques were employed in order to facilitate exploration of the relative merits of each approach. II. EXPERIMENT Both Si 111 and c-plane sapphire c-al 2 O 3 were used as substrates for the three growth processes. A. MOCVD ZnO was deposited by MOCVD Fig. 1 a in a watercooled vertical quartz reactor with an inner diameter of 40 mm in the growth zone. The Zn source was dimethyl zinc 1678 J. Vac. Sci. Technol. B 27 3, May/Jun /2009/27 3 /1678/6/$ American Vacuum Society 1678

2 1679 Sandana et al.: Comparison of ZnO nanostructures grown using pulsed laser deposition 1679 FIG. 3. Color online SEM images of ZnO grown on Si 111 by MOCVD. B. PVT ZnO was deposited by carbothermal evaporation in a tubular furnace with an inner diameter of 30 mm Fig. 1 b.a 2g 1:1 mass ratio mix of ZnO and graphite powders was used for the growth and covered a 1 cm 2 surface. N 2 was used as the carrier gas at 80 SCCM gas flow rate. Both outlets of the furnace were kept open to ambient air. A ceramic holder with the powder was inserted into the quartz tube when the furnace temperature stabilized at 1100 C. The c-al 2 O 3 and Si 111 substrates were placed with a distance of 5 cm from the end of the powder boat to the beginning of the substrate. The reaction time was 30 min. C. PLD ZnO nanostructures were grown from a 99.99% pure ZnO target by PLD Fig. 1 c using a KrF excimer laser 248 nm as described elsewhere. 11,12 FIG. 1. Growth process schematics of a MOCVD, b PVT, and c PLD. triethylamine DZT CH 3 2 Zn-N CH 2 CH 3 3 and the carrier gas was N 2. The flow rate was 500 SCCM SCCM denotes cubic centimeter per minute at STP for the carrier gas and the DZT combined. N 2 O gas was used as the O source and its flow rate was also 500 SCCM. The substrate was placed in the middle of the reactor on a graphite susceptor, which was inclined at 45 to the vertical. The susceptor was heated to 800 C during film growth using a radio frequency rf coil. III. CHARACTERIZATIONS The sample morphology was studied using a Hitachi S4800 field emission-scanning electron microscope SEM. The crystal quality of the nanostructures was investigated using x-ray diffraction XRD performed in a Panalytical MRD Pro system using Cu K radiation. The x-ray optics and spot size were kept the same for all samples. Optical properties were studied via room temperature photoluminescence PL with a continuous-wave frequency-doubled argon ion laser 244 nm, power of 30 mw. FIG. 2. Color online SEM images of ZnO on c-al 2 O 3 grown by MOCVD. FIG. 4. Color online SEM images of ZnO nanostructures on c-al 2 O 3 grown by PVT. JVST B-Microelectronics and Nanometer Structures

3 1680 Sandana et al.: Comparison of ZnO nanostructures grown using pulsed laser deposition 1680 FIG. 5. Color online SEM images of ZnO nanostructures on Si 111 grown by PVT. FIG. 7. Color online SEM images of ZnO nanostructures grown by PLD on Si 111. IV. RESULTS A. SEM investigations 1. SEM images for MOCVD growth of ZnO SEM images for MOCVD growth of ZnO on c-al 2 O 3. Figure 2 a is a SEM image for a MOCVD growth on c-al 2 O 3. The image shows a forest of microcolumns/wires of rather uniform diameter and length typically 1.5 and 30 m, respectively. The majority of the columns have a preferred orientation perpendicular to the substrate plane. The higher magnification image shown in Fig. 2 b reveals that some of these microstructures are partially hollowed, such that the best description of their form might be hexagonally faceted nanotubes. The origin of this hollowing is still under consideration. SEM images for MOCVD growth of ZnO on Si (111). For growth on Si 111, a wide range of microstructures of varying form and size was obtained. For one particular region expanded in Fig. 3 b, an unusual microrod structure with 12 facets and a structure which resembles bevelling of a table leg were observed. The presence of this atypical microstructure could be related to an effect of the Si substrate, since microstructures on c-sapphire did not show this form or symmetry. A suggested growth process for similar structures reported in the literature 13,14 proposes a that such faceting can result from preferential lateral growth along the , and directions to give the faceted crystallites and b that the bevelling can be the interfaces between many independently formed crystallites which combine along the axis of the rod. FIG. 6. Color online SEM images of ZnO nanostructures grown by PLD on c-al 2 O SEM images for PVT growth of ZnO SEM images for PVT growth of ZnO on c-al 2 O 3. A very wide range of ZnO nanostructures, of varying form and size, were observed for samples grown by PVT on c-al 2 O 3.No preferred orientation was observed. Figure 4 a shows pyramidal, faceted structures. Figure 4 b shows typical nanowire type structures for which a suggested growth mechanism has been proposed elsewhere: 15,16 the reaction of Zn vapor and O form a hexagonal columnar base on the substrate Figure 4 a. On top of this hexagonal base the 0001 plane, a nucleation of ZnO particles can occur, leading to a full ZnO hexagonal columnar pin Fig. 4 b. SEM images for PVT growth of ZnO on Si (111). The growth of ZnO on Si 111 by PVT also presented a very wide range of nanostructures of varying form, size, and orientation. Of particular note was a region, expanded in the SEM image in Fig. 5 a, which showed a remarkable ZnOnanocomb-like structure. A possible growth process for such structures has also been proposed in the literature: 17 at the initial stages of growth, Zn and O combine on the substrate to form a microwire. During subsequent growth, the combined effect of diffusion gradients, different relative growth rates, and thermal perturbations cause inhomogeneous nuclei to form on the surface of the microwire. Nanowires then grow on these nuclei to form the nanocomblike structure. Commonly observed, hexagonally faceted, nanowires were also visible in the same sample Fig. 5 b. 3. SEM images for PLD growth of ZnO SEM images for PLD growth of ZnO on c-al 2 O 3. Figure 6 a shows a typical region of sample for the ZnO nanostructures grown on c-al 2 O 3 by PLD. The image shows a high density array of nanostructures of rather similar shape with a strong preferred orientation perpendicular to the substrate plane. The higher magnification image in Fig. 6 b reveals that some of these nanostructures look something like incomplete nanotubes. The growth process for such nanostructures is unclear but the structures indicate a preferential growth along both the c-axis and one basal plane axis. 16 SEM images for PLD growth of ZnO on Si (111). Inthe SEM image shown in Fig. 7 a, an array with a very high density of nanostructures can be seen. The higher magnification image of Fig. 7 b reveals nanorods of rather uniform shape, typically 200 nm in diameter and 3 m long. The vast J. Vac. Sci. Technol. B, Vol. 27, No. 3, May/Jun 2009

4 1681 Sandana et al.: Comparison of ZnO nanostructures grown using pulsed laser deposition 1681 FIG. 8. a Omega scans and b two theta omega scans for ZnO nanostructures grown by MOCVD on c-al 2 O 3. FIG. 9. a Omega scans and b two theta omega scans for ZnO nanostructures grown by PVT on c-al 2 O 3. FIG. 10. a Omega scans and b two theta omega scans for ZnO nanostructures grown by PLD on c-al 2 O 3 and Si 111. JVST B-Microelectronics and Nanometer Structures

5 1682 Sandana et al.: Comparison of ZnO nanostructures grown using pulsed laser deposition 1682 TABLE I. Comparison of XRD scans intensities and - for ZnO nanostructures grown on c-al 2 O 3 Intensity counts/s cps rocking curve deg MOCVD ZnO/c-Al 2 O PVT ZnO/c-Al 2 O PLD ZnO/c-Al 2 O majority of the columns is strongly aligned along the perpendicular to the substrate plane. It has been suggested that the shape of such nanostructures could be explained by a preferential hexagonal growth along the c-axis plus a secondary preferential growth along the 1011 axis related to different relative cyrstal growth rates. 16 The relatively homogeneous array of vertically aligned nanorods could be useful for many potential applications such as improved light extraction in LEDs or directional sensitive detectors. B. XRD investigations Strong ZnO 0002 reflections corresponding to a c-axis oriented wurtzite phase were observed for the samples grown on c-al 2 O 3 by all three growth process Figs The intensity of the XRD peak for the nano-zno grown by PLD Tables I and II is more than three orders of magnitude higher than those for the structures grown by MOCVD and PVT. Although direct comparison of XRD peak intensity cannot be absolute, SEM study suggested that the volume of ZnO was comparable for all the samples, so the PLD samples appear to be much better crystallized. The rocking curve full width at half maximum was smallest for the nanostructures grown by PLD and largest for those grown by MOCVD. For the Si 111 substrate, only the nanostructures grown by PLD gave a response in the XRD analysis Fig. 10 and Table II. This indicates a relatively poor crystallization on Si 111 compared with that on c-al 2 O 3. The c-axis lattice constants of the nanostructures were determined from the ZnO 0002 peak position and found to be similar for all the nanostructures from to Å and close to that for relaxed wurtzite ZnO. 18 In summary, the nanostructures grown by PLD appeared to be better crystallized and have less dispersion in crystallographic orientation than the other nanostructures. FIG. 11. Color online PL spectra for ZnO nanostructures grown on Si 111. C. PL investigations PL spectra for all samples Figs. 11 and 12 and Tables III and IV showed an ultraviolet UV band and a green band N.B. there is a gap in all spectra at around 488 nm due to the second harmonic peak of the UV laser 244 nm used for this experiment. The UV emission was indexed as ZnO near band edge NBE emission 19 and the green emission was attributed to defects in the ZnO. 20 The NBE emission wavelength and were lower for the structures grown on Si substrates than for those grown on c-al 2 O 3. The NBE emission wavelengths max were also observed to depend on growth technique. The structures grown by PLD had the shortest NBE max nm on Si and nm on c-al 2 O 3 and the structures grown by PVT had the longest max nm on Si and nm on c-al 2 O 3. Structures grown by MOCVD had NBE max at nm on Si and nm on c-al 2 O 3. The intensity of the PL peak for the nano-zno grown on Al 2 O 3 and by MOCVD Table IV is more than an order of magnitude higher than those for the structures grown by PLD and PVT. The lower NBE max and smaller for structures grown on Si compared to those grown on c-al 2 O 3 could be related to Al diffusion from the c-al 2 O 3 substrate, which the authors have observed by secondary ion mass spectroscopy in ZnO thin films grown at similar temperatures. 21 V. CONCLUSION ZnO nanostructures were grown on Si 111 and c-al 2 O 3 substrates by MOCVD, PVT, and PLD. The comparison of nanostructures grown with these three different techniques was complicated by the fact that they gave structures with TABLE II. Comparison of XRD scans intensities and - for ZnO nanostructures grown on Si 111. Intensity counts/s cps rocking curve deg MOCVD ZnO/Si 111 No peak No peak PVT ZnO/Si 111 No peak No peak PLD ZnO/Si J. Vac. Sci. Technol. B, Vol. 27, No. 3, May/Jun 2009 FIG. 12. Color online PL spectra for ZnO nanostructures grown on c-al 2 O 3.

6 1683 Sandana et al.: Comparison of ZnO nanostructures grown using pulsed laser deposition 1683 TABLE III. Comparison of PL spectra, main peak,, and intensity for ZnO nanostructures grown on Si 111. Intensity a.u. PLD ZnO/Si MOCVD ZnO/Si PVT ZnO/Si different forms and scales. Indeed, SEM revealed a myriad of ZnO nanostructures such as hexagonal nanorods, nanoneedles, and nanotubes along with novel bevelled structures with 12 facets. PVT growth gave the biggest family of nanostructures. A dense array of regular nanorods with a preferred orientation perpendicular to the substrate plane was obtained on both Si and c-al 2 O 3, by PLD, without the use of a catalyst. XRD did not reveal peaks for the structures grown on Si by MOCVD and PVT. XRD scans for the PLD nanostructures grown on c-al 2 O 3 gave a much more intense 0002 peak and the smallest rocking curve, suggesting that the PLD structures were very well crystallized and the most highly oriented. PL spectra showed that the nanostructures grown TABLE IV. Comparison of PL spectra, main peak,, and intensity for ZnO nanostructures grown on c-al 2 O 3. Intensity a.u. PLD ZnO/c-Al 2 O MOCVD ZnO/c-Al 2 O PVT ZnO/c-Al 2 O by PLD had the lowest max and the smallest. This is consistent with MOCVD having higher impurity doping levels than the PLD. Structures grown on Si had lower max and smaller than those grown on c-al 2 O 3. This redshift and peak broadening on c-al 2 O 3 may be related to Al diffusing into the ZnO from the substrate. 1 D. J. Rogers et al., Appl. Phys. Lett. 91, M. Zerdali, S. Hamzaoui, F. Hosseini Teherani, and D. Rogers, Mater. Lett. 60, V. E. Sandana et al., Proc. SPIE 6895, J. Zhong, H. Chen, G. Saraf, Y. Lu, C. K. Choi, and J. J. Song, Appl. Phys. Lett. 90, T. Reuters, Phys. World 21, Ü. Özgür, Ya. I. Alivov, C. Liu, A. Teke, M. A. Reshchikov, S. Doğan, V. Avrutin, S.-J. Cho, and H. Morkoç, J. Appl. Phys. 98, H. J. Fan, P. Werner, and M. Zacharias, Biophys. J. 2, Z. L. Wang, J. Phys.: Condens. Matter 16, R Z. Li, R. Yang, M. Yu, F. Bai, C. Li, and Z. L. Wang, J. Phys. Chem. C 112, X. Y. Kong and Z. L. Wang, Nano Lett. 3, D. J. Rogers et al., Phys. Status Solidi C 5, R. Nishimura, T. Sakano, T. Okato, T. Saiki, and M. Obara, Jpn. J. Appl. Phys. 47, V. A. Coleman, J. E. Bradby, C. Jagadish, and M. R. Phillips, Appl. Phys. Lett. 89, Z. Li, F. Xu, X. Sun, and W. Zhang, Cryst. Growth Des. 8, H. Hou, Y. Xiong, Y. Xie, Q. Li, J. Zhang, and X. Tian, J. Solid State Chem. 177, G. Z. Wang, Y. Wang, M. Y. Yau, C. Y. To, C. J. Deng, and D. H. L. Ng., Mater. Lett. 59, X. Tian, F. Pe, J. Fe, C. Yang, H. Luo, D. Luo, and Z. Pi, Physica E 31, H. Karzel et al., Phys. Rev. B 53, Y. C. Kong, D. P. Yu, B. Zhang, W. Fang, and S. Q. Feng, Appl. Phys. Lett. 78, S. A. Studenikim, N. Golego, and M. Cocivera, J. Appl. Phys. 84, D. J. Rogers et al., Proc. SPIE 7217, 72170F JVST B-Microelectronics and Nanometer Structures